Charge induced deformation of scanning electrolyte before contact

Abstract

The recent developments of scanning electrochemical probe techniques focus on the strategy of scanning electrolyte. For example, scanning electrochemical cell microscopy (SECCM) is based on holding the electrolyte in a glass capillary, while scanning gel electrochemical microscopy (SGECM) immobilizes the gel electrolyte on micro-disk electrodes or etched metal wires. In both SECCM and SGECM, the first and essential step is to approach the electrolyte probe to be in contact with the sample, which is very often achieved by current feedback with a constant applied potential between the probe and the sample. This work attempts to theoretically analyse the deformation of electrolyte during this approaching process. For liquid electrolyte in SECCM, surface tension is considered to counterbalance the gravity and electrostatic force in 2D cylindrical coordinates with axial symmetry. The deformation at equilibrium is solved under certain conditions. For gel electrolyte, a viscoelastic gel is analysed with simplified 1D geometry. Both equilibrium and dynamic approaching are considered. The results suggest that for both liquid and gel electrolytes, critical conditions exist for breaking the equilibrium. When applied potential is higher or the distance is lower than the threshold, the force will not equilibrate and the electrolyte will deform until contact. The critical condition depends on the properties (surface tension for liquid, elastic and viscous modulus for gel) and geometry (radius of capillary for liquid, thickness for gel) of electrolyte. Prospects of further extending the work closer to real experimental scenarios, especially SGECM, are also discussed.